The object of the proposed investigation is to introduce into blood bank methodology a more sensitive technique for resolving problems of blood transfusion compatibility. The standard serologic compatibility test ("crossmatch") detects donor-recipient incompatibility indicated by hemolytic and/or hemagglutinating antibody in recipient serum. There are well documented limitations to this methodology which are (1) intrinsic to hemagglutination assays in general and (2) the consequence of the remoteness of the serologic events from the primary cellular mechanisms (recipient lymphocyte-donor red cell antigen) involved in transfusion recipient alloimmunization. In the proposed study, the hemolytic plaque assay (HPA) will be adapted to resolve selected compatibility problems with a degree of immunologic sensitivity not attainable with hemagglutination methods. By evaluating recipient alloimmunization by the HPA, first in simulated (in vitro) compatibility problems and subsequently in an actual limited clinical (in vivo) trial, the study will document the suitability of this sensitive new immunologic methodology for the resolution of practical transfusion problems.